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    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <!-- <link rel="stylesheet" href="demo1.css"> -->
    <!-- <link rel="stylesheet" href="demo2.css"> -->
    <!-- <link rel="stylesheet" href="demo3.css"> -->
    <style>
        div {
            border: solid 1px #ddd;
            width: 150px;
        }

        div>input[type="text"] {
            border: none;
            outline: none;
        }

        div>input[type="text"]+span:after {
            content: "\21AA";
            font-size: 14px;
            cursor: pointer;
        }
    </style>
    <title>选择器</title>
</head>
<body>
    <div class="first">
        <p>春节终于要来啦</p>
        <p>春节的活动</p>
        <div>
            <p>应该会留下来做科研吧，现在复习前端的基础知识，希望这些知识我能做出来</p>
        </div>
    </div>
    <p id="reference" class="ref">学习的文档是<span class="info">后盾人</span>提供的教程，版权属于人家哈</p>
    <h1 name="hello">temp</h1>
    <h2 name="hello worlda">temp</h2>
    <h2 name="hello-world">temp</h2>
    <h2 name="helloworld">temp</h2>
    <a href="https://www.baidu.com">百度一下，你就知道</a>
    <a href="https://www.sina.com">百度一下，你就知道</a>
    <input type="text">
    <a href="#hdcms">hdcms</a>
    <section></section>
    <section id="hdcms">
        hdcms内容管理系统
    </section>
    <article>
        <span>houdunren.com</span>
        <aside>
            <p>houdunwang.com</p>
            <span>hdcms.com</span>
            <aside>
                <!-- 这里span类型出现了两次 -->
                <span>aaa</span>
                <span>bbb</span>
            </aside>
        </aside>
    </article>
    <table border="1">
        <tr>
          <td>houdunren.com</td>
          <td>hdcms.com</td>
          <td>后盾人</td>
          <td>houdunwang.com</td>
          <td>hdcms</td>
        </tr>
    </table>
    <ul>
        <li>houdunren.com</li>
        <li>hdcms.com</li>
        <li>后盾人</li>
    </ul>
    <form>
        <input type="email">
        <input type="text" name="title" required>
        <input type="text" name="name">
        <input type="text" disabled>
        <input type="text" name="info">
        <button>保存</button>
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        <input type="text"><span></span>
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